U.S. patent number 4,614,040 [Application Number 06/576,024] was granted by the patent office on 1986-09-30 for borehole survey system and method of determining the difference in borehole azimuth at successive points.
This patent grant is currently assigned to Sunstrand Data Control, Inc.. Invention is credited to Rand H. Hulsing, II, Rex B. Peters, Kurt E. Steinke.
United States Patent |
4,614,040 |
Hulsing, II , et
al. |
September 30, 1986 |
Borehole survey system and method of determining the difference in
borehole azimuth at successive points
Abstract
A borehole survey instrument has a probe with a polarized light
system for transmitting a signal representing the angular
orientation of the probe to the surface. Light from a source in the
probe is directed through a polarizing filter with an axis of
polarization orthogonal to the longitudinal axis of the probe. The
polarized light beam is transmitted to the surface through an
optical fiber light conduit. The angle of polarization is detected
with a rotating polarizing filter and provides a measure of the
probe orientation. In surveying a borehole, azimuth is determined
from inclinometer measurements. The probe orientation in vertical
sections of the borehole is measured by the polarized light system.
Two measures of borehole azimuth are combined, providing an
improved measure of azimuth in boreholes near vertical and near
horizontal.
Inventors: |
Hulsing, II; Rand H. (Redmond,
WA), Peters; Rex B. (Woodinville, WA), Steinke; Kurt
E. (Bellevue, WA) |
Assignee: |
Sunstrand Data Control, Inc.
(Redmond, WA)
|
Family
ID: |
27019516 |
Appl.
No.: |
06/576,024 |
Filed: |
February 1, 1984 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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406431 |
Aug 9, 1982 |
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Current U.S.
Class: |
33/312 |
Current CPC
Class: |
G01B
11/26 (20130101); E21B 47/135 (20200501); E21B
47/022 (20130101) |
Current International
Class: |
E21B
47/12 (20060101); E21B 47/02 (20060101); E21B
47/022 (20060101); G01B 11/26 (20060101); G01C
019/00 () |
Field of
Search: |
;33/312,313,311,304,302
;73/151 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2039371A |
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Aug 1980 |
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GB |
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2103793A |
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Feb 1983 |
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GB |
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Primary Examiner: Martin, Jr.; William D.
Attorney, Agent or Firm: Wood, Dalton, Phillips, Mason &
Rowe
Parent Case Text
This is a division of application Ser. No. 406,431 filed Aug. 9,
1982.
Claims
We claim:
1. A borehole survey system, comprising:
a borehole probe movable through the borehole;
an inclination sensor in said probe;
means connected with said sensor for deriving a first measure of
borehole azimuth with a high degree of accuracy in a vertical
borehole and a low degree of accuracy in a horizontal borehole;
means connected with said sensor for deriving a second measure of
borehole azimuth with a high degree of accuracy in a horizontal
borehole and a low degree of accuracy in a vertical borehole;
and
means for combining said first and second borehole azimuth measures
in accordance with the borehole inclination.
2. The borehole survey system of claim 1 in which the means for
deriving the first measure of borehole azimuth includes means for
measuring the rotational orientation of the probe in the
borehole.
3. The borehole survey system of claim 1 in which the means for
deriving the second measure of borehole azimuth includes first and
second flexibly connected probe sections and means for measuring
the angle between the probe sections.
4. The borehole survey system of claim 1 in which the means for
combining the first and second borehole azimuth measures
establishes an average of the two azimuth measures weighted in
accordance with borehole inclination.
Description
This invention relates to a borehole survey system and to a method
of determining borehole azimuth.
This application is related to Liu Ser. No. 200,096 filed Oct. 23,
1980, replaced by Ser. No. 428,298 filed Sept. 29, 1982, and
Hulsing Ser. No. 224,789 filed Jan. 13, 1981, now U.S. Pat. No.
4,399,692. The disclosures of both applications are incorporated
herein by reference.
A typical borehole survey instrument has a probe housing which is
suspended from a cable and moved through the borehole. An
inclinometer, for example an orthogonal triad of accelerometers,
measures the angle of the local vertical with respect to the probe.
The probe is free to rotate about its longitudinal axis as it moves
through the borehole. It is necessary to measure the probe
orientation to provide a reference for the inclinometer
measurements, in order to determine the borehole azimuth. It is
known to measure orientation with a gyroscope or a magnetometer.
Both have operating limitations which impair their reliability,
degrade accuracy and contribute to high cost.
The Liu and Hulsing applications disclose borehole survey
instruments which use multi section probes together with means for
determining the incremental azimuth changes as the probe moves
through the borehole. These instruements eliminate the gyroscope or
magnetometer but have other disadvantages, including a long probe
dimension and an accumulation of measurement error which reduces
accuracy of measurement.
The present instrument measures probe orientation directly using
polarized electromagnetic radiation and transmits an orientation
signal to the surface through a conduit which maintains the axis of
polarization of the signal. The instrument is particularly useful
in providing a measure of probe orientation as the probe traverses
a vertical borehole section where orientation cannot be measured
with an inclinometer.
A feature of the invention is an improved and simplified method for
determining borehole azimuth from successive inclinometer
measurements.
Another feature of the invention is that the borehole survey system
comprises a probe movable through the borehole, an inclination
sensor in the probe, means for deriving a first measure of borehole
azimuth with a high degree of accuracy in a vertical borehole and a
reduced accuracy in a horizontal probe borehole, means for deriving
a second measure of borehole azimuth with a high degree of accuracy
in a horizontal borehole and a low degree of accuracy in a vertical
borehole and means for combining the first and second borehole
azimuth measures in accordance with the borehole inclination.
Further features and advantages of the invention will readily be
apparent from the following specification and from the drawings, in
which:
FIG. 1 is a diagram of an apparatus embodying the invention,
including a section through the borehole showing the probe;
FIG. 2 is a fragmentary diagrammatic illustration of the probe
showing the polarized light source and inclinometer;
FIGS. 3 and 4 are diagrammatic illustrations of the detection of
the angular orientation of the probe;
FIGS. 5-7 are geometric diagrams used in describing the
determination of borehole azimuth;
FIG. 8 is a diagram of a borehole survey apparatus with means for
deriving and combining two measures of borehole azimuth; and
FIG. 9 is a block diagram of a system for transmitting sensor
information to the surface through amplitude modulation of the
polarized light beam.
In the survey system of FIG. 1, a probe 20 is suspended from a
cable 21 for movement through a borehole 22. The probe 20 is
centered within the borehole by suitable spacers 23 so that the
longitudinal axis of the probe is centered in the borehole and may
be considered coincident with the borehole axis. Probe 20 is free
to rotate as it moves through the borehole. Cable 21 passes over a
rotating wheel which provides a measure l of the distance of the
probe downhole. A cable hoisting mechanism for lowering and raising
probe 20 is not shown to avoid complicating the drawing.
Briefly, probe 20 includes means for measuring the inclination of
the borehole with respect to the vertical or gravity vector at
successive points along the borehole. As will appear, this
measurement in a slant borehole provides sufficient information for
a determination of the change in borehole azimuth from point to
point. Many boreholes have a vertical section, particularly the
initial section below the surface. Orientation of the probe in a
vertical borehole is measured using the polarized light system
described below. Signals representing inclination and orientation
of the probe are transmitted to the surface through cable 21 and
coupled to a detector 25. The output of the detector is in turn
coupled with a data processor 26 connected with a keyboard and
display 27 used to input data to and derive information from the
system.
The elements of the probe relevant to the invention are illustrated
schematically in FIG. 2. The probe has a housing 30 in which is
located an inclinometer 31 which is preferably made up of an
orthogonal triad of accelerometers 32, 33, 34 with their sensitive
axes designed X, Y, Z, respectively. The Z axis is shown as
coincident with the longitudinal axis of the probe. The X and Y
axes define a plane at right angles to the Z axis. The
accelerometers which measure the local gravity vector are
preferably servoed devices. Signals from the accelerometers define
the inclination angle of the borehole from the vertical and in a
slant hole the rotation angle of the probe with respect to a
vertical plane through the probe axis. Two accelerometers or other
angular relationships could be used, but the illustrated
inclinometer is preferred.
A light source 35, as a light emitting diode, is located at the
upper end of the probe 20. A polarizing filter 36 is fixed in
housing 30 to polarize the light from source 35 along an axis
transverse to the longitudinal axis of the probe. An optical fiber
light conduit 37 receives polarized light from filter 36 and
conducts it to the surface. Optical fiber conduit 37 may be
incorporated in hoisting cable 21. The end 37a of the optical fiber
light conduit is preferably fixed to the end of probe housing 30.
If, however, rotation of probe 20 as it moves through the borehole
causes excessive twisting of cable 21, the cable and optical fiber
light conduit 37 can be connected with the probe housing 37 through
a swivel joint (not shown).
As probe 20 rotates about its axis, the plane of polarization of
the light from LED 35 rotates with the probe. The plane of
polarization established by filter 36 is not substantially modified
either by reflection of the light as it passes through the fiber
optic light conduit 37 or by twisting of cable 21 and the light
conduit. Accordingly, the axis of polarization of the light
detected at the surface represents the orientation of the probe
about its longitudinal axis.
The polarized light received at the surface through optical fiber
light conduit 37 is directed to a light sensor 40 which is
connected with detector 25. Interposed between the optical fiber
light conduit and sensor 40 is a second polarizing filter 41 which
is rotated by a motor 42.
The determination of the probe orientation from the polarized light
is illustrated in FIGS. 3 and 4. As filter 41 rotates, the light
received by sensor 40 is a maximum when the polarization axes of
the filters are coincident and a minimum when the axes are
90.degree. displaced. The curves in FIGS. 3 and 4 plot the received
light or sensor signal amplitude as a function of the angular
position of filter 41. In FIG. 3 the polarization axis of filter 36
is aligned with that of filter 41 at the 0.degree. position. Signal
maximums occur at 0.degree. and 180.degree.. Signal minimums occur
with filter 41 at 90.degree. and 270.degree.. In FIG. 4, probe 20
is displaced 90.degree. from its rotational position in FIG. 3.
With filter disc 41 at the 0.degree. position, the signal amplitude
is minimum and this condition is repeated with the filter disc at
180.degree.. Signal maximums occur at 90.degree. and
270.degree..
In surveying a borehole, the probe 20 is oriented to a known
azimuth reference at the top of borehole 22. The angular relation
between the output of sensor 40 and rotating filter 41 is noted. As
the probe moves through the borehole rotational orientation of the
probe is correlated with the distance l of the probe along the
borehole. It is necessary only that the angular velocity of the
rotating filter 41 be much greater than the angular velocity of the
probe 20.
Signals representing the output of sensor 40 and the angular
position of filter disc 41 are coupled with detector 25 which
measures the phase angle of the signal with respect to the angular
position of filter 41 and determines the rotational position of
probe 20. The relative difference between signal peaks and nulls
may vary with system conditions, but the phase angle does not. The
signal is basically a half-wave rectified sine curve with a DC
bias. Detector 25 may, for example, incorporate a data processor
which applies a Fourier curve fit to the detector signal. The
fundamental frequency component of the signal is twice that of the
wheel rotation. Variable terms in the Fourier representation of the
signal can be ignored. The phase angle of the signal uniquely
identifies probe orientation.
The determination of the downhole position of the probe from the
signals representing orientation of the probe about its
longitudinal axis, distance downhole and the orientation of the
probe with respect to gravity from inclinometer 31 will be
described in connection with FIGS. 5, 6 and 7. In FIG. 5, borehole
22 is depicted extending downwardly from the surface of the earth.
A three-dimensional coordinate system, N (north), E (east), G
(gravity) has its origin at the intersection of the borehole with
the surface. The local surface area may be considered planar. With
the probe 20 located at a point i inclinometer 31 measures the
angle .psi. between the transverse reference axis of the probe,
i.e., the axis of polarization of filter 36, and the vertical plane
45 which contains the longitudinal axis of the probe. The angle
.psi. is indicated between the polarization axis of filter 36
(sometimes referred to as the transverse reference axis of the
probe) and a line 46 normal to the longitudinal axis of the probe
and lying in vertical plane 45. The inclination angle I of the
probe and borehole at point i is shown as the angle between an
extension of the longitudinal axis 47 of the probe and the vertical
line 48 in plane 45.
The intersection of vertical plane 45 with the surface of the earth
defines a line 49 the orientation of which is the azimuth of the
borehole at point i. The azimuth angle A is measured clockwise from
north, looking down on the surface of the earth.
The inclination angle I is calculated from the accelerometer
signals of inclinometer 31. Similarly, at any point where the
borehole axis is not vertical, the angle .psi. is calculated from
the accelerometer signals. Where, however, the borehole axis is
vertical, there is no unique vertical plane and the angle .psi. not
defined. A typical borehole has an initial vertical section and it
is in traversing such a vertical section that the polarized light
rotational orientation detector is used. As described above, the
start of a survey operation, the probe orientation and the phase
angle of the polarized light signal are noted. Changes in the
rotational orientation of the probe as it is lowered through a
vertical borehole section are recorded. When the probe leaves the
vertical borehole section, its orientation is known and provides
the basis for further determination of borehole azimuth.
It was pointed out above that the angle .psi. is determined from
the accelerometer signals. In the Liu application identified above,
there is a disclosure that the angle .psi. and borehole azimuth are
related. The relationship is implicitly involved in a series of
matrix operations by which Liu derives a representation of the
borehole trajectory. An explicit expression of the relationship is
the basis of the method of determining borehole azimuth in
accordance with the present invention.
In FIG. 7, two successive borehole points i and i+1 are shown and
it is assumed that the borehole section between the two points is a
plane curve. This is not always true in a borehole but is a
reasonable approximation and may be made as accurate as desired by
selecting very small distances between points. The plane P.sub.i is
a vertical plane containing the longitudinal axis of the probe at
the point i. Plane P.sub.i has an azimuth angle A.sub.i and the
inclination of the probe at point i is I.sub.i. Plane P.sub.i+1 is
a vertical plane containing the longitudinal probe axis at point
i+1. The plane R contains the plane borehole curve from point i to
point i+1.
The vector j is a unit vector in the probe coordinate system,
initially aligned with the E axis of the global coordinate system.
It can be shown using the method of Euler angles that the
components of the unit vector j in the global coordinate system
are:
N=-cos A cos I sin .psi.-sin A cos .psi.
E=-sin A cos I sin .psi.+cos A cos .psi.
G=sin I sin .psi.
In FIG. 7, the unit vector j is shown at both points i and i+1,
rotated through an angle .theta. from the respective vertical
planes P.sub.i and P.sub.i+1 so that the vectors are perpendicular
to the plane of the borehole curve at both points. The two units
vectors will then have the same direction and thus the same
components in the NEG coordinate system. Therefore, ##EQU1##
The change of azimuth angle from point i to i+1 may be expressed
as
By linear combination of the N and E equations, two new equations
may be derived in terms of .DELTA..sub.i, ##EQU2##
An angle .gamma..sub.i is defined as
The equations above may be combined with the equation for the G
components of the unit vector, ##EQU3##
Solving these equations for sin .DELTA..sub.i and cos .DELTA..sub.i
and dividing one by the other to get tan .DELTA..sub.i, the
following relationship is derived: ##EQU4## Thus, the change in
azimuth angle between successive points of the borehole may be
determined from the inclinometer measurements at the two
points.
As pointed out above, the orientation of probe 20 is measured
directly at the earth's surface. The probe is then lowered through
the borehole. So long as the borehole axis is vertical, the probe
orientation about its longitudinal axis is measured utilizing the
polarized light system. At the point the borehole deviates from the
vertical, changes in azimuth are determined by successive
computations of .DELTA..sub.i and the azimuth angle at any point
determined by adding the azimuth angle increments. Should the probe
encounter another vertical borehole section, the orientation of the
probe about its longitudinal axis as it passes through the vertical
section is monitored by the polarized light system.
If an additional assumption is made that the borehole curve between
points is smooth and if the points are selected to be very close
together so that .psi..sub.i -.psi..sub.i+1 is a small angle, and
##EQU5## This relationship is useful in visualizing behavior of the
survey system and is sufficiently accurate for actual surveying in
some applications.
The apparatus which has been described is simpler than that of the
Liu application in that it utilizes only one inclinometer rather
than two and the probe is a single compact housing rather than two
housings joined by a connection which is flexible to bend along the
axis of the borehole but which resists rotation between the
housings about the borehole axis. These are significant differences
from a mechanical standpoint. However, a more important difference
in the present borehole survey instrument and method is in the
nature of the derivation of the borehole azimuth. In the prior Liu
and Hulsing systems measurement errors are cumulative so that the
accuracy of the measurements diminishes as more measurements are
made. In the present system, there is a cancellation of errors so
that the error in any azimuth measurement is a function of the
difference between the initial azimuth and the final measure. Tens
of thousands of measurements may be made in surveying a borehole so
that the difference in accuracy of the two systems is
significant.
Another important difference is that the accuracy of the Liu and
prior Hulsing systems diminish in vertical or near-vertical
boreholes. In the present system, the initial azimuth measurement
may be made quite accurate and the polarized light system for
measuring probe rotation minimizes errors introduced while the
probe traverses a vertical borehole.
The accuracy of the present system diminishes, however, as the
borehole approaches the horizontal where cos I goes to zero. If a
borehole with a horizontal section is to be surveyed, the
instrument of FIG. 1 is combined with the instrument of Hulsing
U.S. Pat. No. 4,399,629, as shown in FIG. 8. Here, probe 55 is
suspended from cable 56 in borehole 57. Probe 55 has two sections
58, 59 connected by a flexible joint 60 of the character described
in U.S. Pat. No. 4,399,629. Upper probe section 58 houses an
inclinometer and a polarized light source as in FIG. 2. Flexible
joint 60 is provided with means for generating signals representing
the angle between the two probe sections. The various signals are
transmitted to the surface through cable 56 and are coupled with
receiver, detector and processor, block 62. The polarized light
system and inclinometer signals are processed to develop a first
measure of azimuth A which has a high degree of accuracy in a
vertical borehole. Signals from the inclinometer and from joint 60
are processed to develop a second measure of azimuth A' which has a
high degree of accuracy in a horizontal borehole and a lesser
degree of accuracy in a vertical borehole. The azimuth signals A
and A' are combined in averaging circuit 63 in accordance with the
probe inclination I to produce a composite azimuth signal A.sub.ave
where
Signals from the various sensors in the probe are preferably
transmitted to the surface in digital form by amplitude modulation
of the polarized light beam. The system for accomplishing this is
illustrated in block form in FIG. 9. The various sensors, e.g.,
accelerometers 32, 33 and 34 and the angle sensors of joint 60,
FIG. 8, are represented at block 65. The outputs of the sensors are
selected individually by a multiplexer and converted from analog to
digital form at block 66. The serial digital signals are coupled to
lamp 35 and modulate the intensity of the light beam. The signal
from light sensor 40 has the waveform illustrated at 67 although in
practice the repetition rate of the digital signals may be many
times that illustrated. The signal from light sensor 40 is
connected with both the probe angle detector 68 and a digital
signal detector 69. The outputs of the detectors are connected with
processor 70. If the relative amplitudes of the rectified sine wave
and the digital pulses are such that the digital signals are lost
at the null of the analog signal, the digital data may be read only
at the peaks of the sine waves. In this situation the digital
signals representing each sensor output may be repeated to avoid
loss of sensor information.
* * * * *